4. Nitrogen: Nitrogen (78 per cent) is present in all living organisms and
helps in the growth of living beings.
Oxygen: It is a colourless and tasteless gas, and is the second most
prevalent gas in the atmosphere (21 per cent). Living beings breathe it
to live. Oxygen in the form of ozone (O3) is present in the earth’s
atmosphere and helps to protect the earth from ultraviolet radiation
from the sun.
Carbon dioxide: Carbon dioxide exists in the earth’s atmosphere as a
colourless and odourless gas. It is produced by all animals, fungi and
microorganisms during respiration and is used by plants during
photosynthesis. Carbon dioxide helps in creating the greenhouse
effect. Without the greenhouse effect, it would be too cold for life to
exist on this planet.
Water vapour: Water vapour is constantly generated by evaporation
and removed from the atmosphere by condensation. Atmospheric
water vapour falls on earth as rain, which is very important for our
agriculture and is also a source of fresh water.
7. Stratosphere
The temperature is constant in the lower part but increases in the
upper part. It is about 0°C at 50 km. This increase in temperature
is because of the absorption of the sun’s harmful ultraviolet (UV)
rays by the ozone gas present in this layer. However, the
temperature begins to fall again. The stratosphere is almost
without clouds because of the extremely low temperatures in the
tropopause due to which there is hardly any movement of water
vapour from the troposphere to the stratosphere. The
stratosphere is characterized by an increase in ozone gas with a
maximum ozone concentration at about 30 km. The stratosphere
is, therefore, also called the ozonosphere because of this
presence of the ozone layer. This ozone layer protects us from
the sun’s ultraviolet rays.
Commercial airlines normally fly in the lower levels of the
stratosphere. They do this to avoid atmospheric turbulence that
occurs in the troposphere. The stratopause, the upper part of this
layer, separates it from the mesosphere
10. It is the uppermost layer of the atmosphere. It
starts at the top of the thermosphere and
continues until it merges with space.
Temperatures increase with altitude in this
layer because of solar radiation and can
exceed 4,000°C. While hydrogen and helium
are the main gases of this layer, they exist
only in very small quantities. Structure of the
atmosphere This is because of the absence of
gravity.
